1. Field of the Invention
The present invention relates to an apparatus for and a method of capturing a radiation image by emitting radiation from a radiation source and applying the emitted radiation to a subject, and for controlling the radiation source based on the dose of radiation applied to the subject.
2. Description of the Related Art
In the medical field, for example, it has been customary to apply radiation to a subject and to detect the amount of radiation that has passed through the subject with a radiation detector, or to guide the radiation that has passed through the subject directly to an X-ray film or the like, thereby forming a radiation image of the subject for diagnostic purposes.
For obtaining a radiation image suitable for image interpretation and diagnosis, a tube voltage, a tube current, and a radiation application time are established as appropriate exposure conditions, depending on the body region to be imaged and other characteristics of the radiation source. There is known an image capturing system with an automatic exposure control function, which controls the dose of radiation to be applied to a subject, by detecting the dose of radiation that has passed through the subject with a dose detector, and then automatically stopping application of radiation when the detected dose reaches a predetermined value (see Japanese Laid-Open Patent Publication No. 2004-298383).
In the image capturing system having such an automatic exposure control function, a small current output from the dose detector is converted into a voltage signal by a current-to-voltage converter. The voltage signal is amplified at a high magnification by an amplifier, and the amplified voltage signal is integrated with respect to time by an integrator, thereby determining a radiation dosage.
However, since the radiation dosage is determined after a low voltage signal has been amplified at a high magnification, the image capturing system having such an automatic exposure control function is problematic in that a temperature-dependent characteristic change of the circuit components tends to cause a large error in the determined radiation dosage.
In order to compensate for such temperature-dependent characteristic changes of the circuit components, there has widely been employed a process of canceling out the characteristic circuit component changes using a temperature compensating device, whose input/output characteristics change depending on temperature, such as a thermistor, a diode, or the like, wherein the temperature compensating device is inserted into the system circuitry (see Japanese Laid-Open Patent Publication No. 5-87607 and Japanese Laid-Open Patent Publication No. No. 5-299955).
The temperature characteristics of the temperature compensating device vary from unit to unit. Therefore, if the temperature compensating device is not thermally coupled adequately to the circuit component whose temperature-dependent characteristic change is to be compensated for, then the temperature compensating device cannot provide highly accurate temperature compensation. In addition, the circuit component has its own time-variable characteristics, and as a practical matter, it is difficult to select a temperature compensating device that is capable of handling variations in characteristics.